Iron powder compositions

ABSTRACT

The invention concerns a method of improving the dynamic properties of compacted and sintered products having a density between 6.8 and 7.6 g/cm 3 , preferably between 7.0 and 7.4 g/cm 3 . According to this method an iron based powder, graphite and a solid particular lubricant having a vaporizing temperature less the sintering temperature, preferably less than about 800° C. is compacted and sintered and the maximum particle size of the lubricant is selected so that the largest pores of a compacted and sintered product prepared from the composition are equal to or less than the largest pores obtained in a compacted and sintered product prepared from the composition without lubricant. The invention also concerns composition of an iron based powder, graphite and a solid particular lubricant having a vaporizing temperature less the sintering temperature, preferably less than about 800° C. and a maximum particle size less than about 0.3 of the maximum size of the iron based powder as measured by laser diffraction measurement.

FIELD OF THE INVENTION

[0001] The present invention concerns iron based powder compositions forthe preparation of compacted and sintered products having improvedproperties. More specifically the invention concerns the influence ofthe largest particles of the lubricant and the iron based powder used inthe composition on the dynamic properties of the final products.

BACKGROUND OF THE INVENTION

[0002] Fatigue performance of sintered steels are influenced by severalfactors which are interacting. The density was early established as oneof the most influential factors together with the microstructure andalloy element content but also homogeneity, pore size and pore shape areknown to influence the dynamic properties. This makes fatigueperformance one of the most complex properties of PM materials.

OBJECTS OF THE INVENTION

[0003] An object of the present invention is to improve the dynamicproperties of sintered steels, specifically sintered steels having adensity between 6.8 and 7.6 g/cm³.

[0004] Another object of the invention is to eliminate the influence ofthe particle size of the lubricant on the dynamic properties, especiallythe fatigue strength of the sintered parts.

[0005] A third object is to provide a method of improving the fatiguestrength by selecting the particle size of the lubricant in view of theparticle size of the iron powder.

SUMMARY OF THE INVENTION

[0006] According to the invention it has now been found that, even ifthe amount of the very largest particles of a lubricant constitutes anegligible or almost negligible fraction of the lubricant particle sizedistribution as well as of the amount of the lubricant, this fractionhas an unexpectedly large detrimental effect on the pore size andaccordingly on the dynamic properties.

[0007] Similarly it has been found that the very largest particles ofthe iron powder, i.e. the maximum size of the iron powder has anunexpectedly large detrimental effect on the dynamic properties. Thus inorder to get improved dynamic properties the maximum size of thelubricant particles as well as the maximum size of the iron powdershould be reduced. For presently commercially used ferrous based presspowders this means that the maximum particle size of the lubricantshould be less than about 60 μm as measured by laser diffractionmeasurement.

[0008] In order to achieve the best dynamic properties for a given ironbased powder (at a given density) a relationship between the maximumsize of the particles of the lubricant and the maximum size of theparticles of the iron based powder has also been established. The term“maximum size” as used in this context is defined in the formula below.

[0009] In accordance with the invention it has been found that theparticle size of the lubricant in a composition including the lubricantand an iron based powder for powder metallurgical preparation ofcompacted and sintered products should be selected so that the largestpores of the compacted and sintered product prepared from thiscomposition should be equal to or less than the largest pores obtainedin a compacted and sintered product prepared from the same compositionwithout the lubricant, which in practice means that the compaction isperformed in a lubricated die.

[0010] Empirically we have found the following relationship between thelargest lubricant particles and the largest iron powder particles inorder to avoid the influence of the lubricant on the size of the largestpores.

Lub _(max)≦0.31×Fe_(max) _(⁻) 26, wherein

[0011] Lub_(max) is the lubricant particle size in μm whereas 99.99% ofthe lubricant is finer.

[0012] Fe_(max) is the iron particle size in μm whereas 99.99% of theiron powder is finer

[0013] (this could also be expressed as Lub_(max) is the size of thelargest one hundredth of a percent fraction of lubricant particles inμm,

[0014] Fe_(max) is the size of the largest one hundredth of a percentfraction of the particles of the iron based composition in μm). Thismeans that the maximum particle size of the lubricant as defined aboveshould be less than about 0.3 of the maximum size of the iron oriron-based particles.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The iron based powder according to the invention may be analloyed iron based powder, such as a prealloyed iron powder or an ironpowder having the alloying elements diffusion-bonded to the ironparticles. The iron based powder may also be a mixture of an essentiallypure iron powder and the alloying elements.

[0016] The alloying elements which can be used in the compositionsaccording to the present invention may be one or more elements selectedfrom the group consisting of Ni, Cu, Cr, Mo, Mn, P, Si, V and W. Theparticle sizes including the maximum particle sizes of the alloyingelements are smaller than those of the iron or iron-based powder. Thevarious amounts of the different alloying elements are between 0 and 10,preferably between 1 and 6% by weight of Ni, between 0 and 8, preferablybetween 1 and S % by weight of Cu, between 0 and 25, preferably between0 and 12% by weight of Cr, between 0 and 5, preferably between 0 and 4%by weight of Mo, between 0 and 1, preferably between 0 and 0.6% byweight of P, between 0 and 5, preferably between 0 and 2% by weight ofSi, between 0 and 3, preferably between 0 and 1% by weight of V andbetween 0 and 10, preferably between 0 and 4% by weight of W.

[0017] The iron based powder may be an atomised powder, such as awateratomised powder, or a sponge iron powder.

[0018] The particle size of the iron based powder is selected dependingon the final use of the sintered product and, according to the presentinvention is has been found that also the maximum particle size of theiron based powder has an unexpectedly large detrimental effect on thedynamic properties of the sintered product.

[0019] The type of lubricant is not critical and the lubricant may beselected from a wide variety of solid lubricants. Specific examples ofsuitable lubricants are conventionally used lubricant such as Kenolube®,Metalub, (both available from H6ganas AB Sweden) H-Wachs® (availablefrom Clariant), and zinc stearate (available from Megret). The amount ofthe lubricant may vary between 0.1 and 2, preferably between 0.2 and1.2. Furthermore the vaporising temperature of the lubricant should bebelow the sintering temperature of the compacted part. Presently usedlubricant which may be used according to the present invention havevaporising temperatures less than about 800° C.

[0020] The amount of graphite varies between 0 and 1.5, preferablybetween 0.2 and 1% by weight of the composition. Also, the maximumparticle size of the graphite powder should be equal to or smaller thanthe maximum particle size of the lubricant.

[0021] In addition to the iron based powder, optional alloying elements,graphite and lubricant(s) the compositions according to the inventionmay also include optional conventionally additives, such as MnS, Mnx™.

[0022] The improved dynamic properties which can be obtained accordingto the present invention are especially interesting in sintered productshaving densities between 6.8 and 7.6 g/cm³, especially between 7,0 and7,4/cm³.

[0023] Examples of preferred iron based powders plus preferred amountsof graphite follows below:

[0024] Iron+4% Ni+1.5% Cu+0.5% Mo where the alloying elements arediffusion bonded to the iron particle mixed with 0.4 to 1% graphite.

[0025] Iron+1.75% Ni+1.5% Cu+0.5% Mo where the alloying elements arediffusion bonded to the iron particle mixed with 0.4 to 1% graphite.

[0026] Iron+5% Ni+2% Cu+1% Mo where the alloying elements are diffusionbonded to the iron particle mixed with 0.4 to 1% graphite.

[0027] Iron prealloyed with 1.5% Mo and 2% Ni diffusion bonded to theiron/Mo particle which are mixed with 0.4 to 1% graphite.

[0028] Iron prealloyed with 1.5% Mo and 2% Cu diffusion bonded to theiron/Mo particle which are mixed with 0.4 to 1% graphite.

[0029] Iron prealloyed with 1.5% Mo and 2% Cu and 4% Ni diffusion bondedto the iron/Mo particle which are mixed with 0.4 to 1% graphite.

[0030] Iron prealloyed with 1.5 or 0.85% Mo and mixed with 0.4 to 1%graphite.

[0031] Iron prealloyed with 3% Cr and 0.5% Mo and mixed with 0.2 to 0.7%graphite.

[0032] These iron based powders all contains powders with a particlesize below 212 μm sieved.

[0033] According to one especially preferred embodiment of the inventionthe maximum particle size of the iron based powder should be less thanabout 220 μm (which is obtained for e.g. Astaloy Mo −106 μm, throughsieve analysis), and for this powder the maximum particle size of thelubricant should be less than 60 μm as measured by laser diffractionmeasurement.

[0034] The compacting and sintering steps for the preparation of thefinal products, which are distinguished by essentially the same orbetter dynamic properties as obtained for the same composition butwithout lubricant are performed under conventional conditions, i.e. thecompac- tion is carried out at pressures between 400 and 1200 MPa andthe sintering is performed at temperatures between 1100 and 1350° C.

[0035] The invention is further illustrated by the follow- ing nonlimiting examples.

EXAMPLE 1

[0036] Five mixes with the same nominal composition were prepared fromDistaloy AE which is a pure iron powder which has 4% Ni, 1.5% Cu and0.5% Mo diffusion annealed to it and which has a main particle sizerange between 20 and 180 μm. The mixes mainly consisted of

[0037] Distaloy AE+0.3% C (UF-4)+0.8% Metalub®

[0038] Distaloy AE+0.3% C (UF-4)+0.8% zinc stearate

[0039] Distaloy AE+0.3% C (UF-4)+0.8% Hoechst wachs®

[0040] Distaloy AE+0.3% C (UF-4)+0.8% Kenolube®

[0041] Distaloy AE+0.3% C (UF-4) (reference, lubricated die)

[0042] The following maximum particles sizes of the lubricants weremeasured by the laser diffraction measurement technique: Maximumparticle Type of lubricant size, μm Metalub ® 147  zinc stearate 73Hoechst wachs ® 51 Kenolube ® 73

[0043] From these mixes 5 TS bars were compacted to a density of 7.10g/cm³. For the mix without lubricant the tool surface was lubricatedwith zinc stearate dispersed in acetone. All bars were sintered at 1120°C. for 30 minutes in endothermic atmosphere with a carbon potentialcorresponding to 0.3% carbon content. After sintering the density,carbon content and pore size distribution were evaluated. Also theparticle size distribution of the different lubricants was measuredusing a Sympatec Helos laser diffraction particle size analysingequipment. The lubricants were dispersed in air for the particlemeasurement.

[0044] The bars manufactured from the different mixes defined above hada very even carbon content and density after sintering. Metallograficsamples were prepared and the pore size distribution was measured on asurface of 25 mm² for every material.

[0045] The relationship between the different lubricants are identicalfor the pore size distribution and the particle size distribution, whichindicates that the size of the largest lubricant particles governs thesize of the largest pores at least for the lubricants containingparticles larger than approximately 60 μm. The pore size distributionfor the lubricated die however shows that the reduction of the internalfriction with addition of lubricants decreases the size of theintermediate porosity. In the case with the lubricant with the smallestcoarse fraction/maximum particle size, lubricant C, the lubricant doesnot at all contribute to the amount of coarse pores. As can be seen fromthe above experiment lubricants with particles larger than 60 μm createscoarse pores in a component made of Distaloy AE+0.5% C at 7.1 9/cm³. Adecrease of the fraction of coarse pores will increase the dynamicalproperties.

[0046] These results demonstrate the possibility to produce a materialwith finer porosity at a given density by using a lubricant, the maximumparticle size of which is determined in view of the maximum particlesize of the iron based powder in accordance with the present invention.

EXAMPLE 2

[0047] The following example illustrates the effect on the fatiguestrength of eliminating the largest particles of the lubricant as wellas the largest particles of the iron based powder.

[0048] The following mixes were used

[0049] Astaloy Mo+0.3% C (UF-4)+0.8% of Hoechst Wachs

[0050] Astaloy Mo(

[0051] 106 μm)+0.3% C (UF-4)+0.8% of Hoechst Wachs.

[0052] Astaloy Mo is a prealloyed material with 1.5% Mo (available fromH6ganas AB, Sweden) which has an approximate particle size rangedistribution of 20-180 μm. The sieved finer grade powder Astaloy Mo

[0053] 106 μm was used to demonstrate the effect of eliminating thelargest particles of the iron based powder. The maximum particle size ofAstaloy Mo as measured by laser diffraction measurement (Sympatec Heloslaser) and the maximum particle size of Astaloy Mo −106 μm were 363 and214 μm, respectively.

[0054] From all materials 20 fatigue test bars and 7 tensile strengthbars were pressed to 7.1 g/cm³ and sintered at 1120° C. for 30 minutesin endothermic atmosphere with a controlled carbon potential. The barswere then evaluated with respect to static properties and fatiguestrength according to the staircase method described in Sonsino C.M.“Method to determine relevant material properties for the fatigue designof powder metallurgy parts”, Powder Metallurgy International 1984 vol.16 p. 34-36. The pore size distribution was evaluated according to themethod described in Example 1.

[0055] The results obtained demonstrate that the products prepared fromthe finer base powder Astaloy Mo (−106 μm) and finer lubricant powderHoechst Wachs have less large pores and an increase of the fatiguestrength of about 15% is obtained with the decreasing fraction of coarsepores. For the tensile strength there is a small increase ofapproximately 5% with decreasing fraction of coarse pores.

1. Method of improving the dynamic properties of compacted and sinteredproducts having a density between 6.8 and 7.6 9/cm³, preferably between7.0 and 7.4 g/cm³, comprising subjecting an iron based powder, graphiteand a solid particular lubricant having a vaporising temperature lessthe sintering temperature, preferably less than about 800° C. to thesteps of compacting and sintering, whereby the maximum particle size ofthe lubricant is selected so that the largest pores of a compacted andsintered product prepared from the composition are equal to or less thanthe largest pores obtained in a compacted and sintered product preparedfrom the composition without lubricant.
 2. Method of improving thedynamic properties of compacted and sintered products having a densitybetween 6.8 and 7.6 g/cm³, preferably between 7.0 and 7.4 g/cm³,comprising the steps of subjecting a mixture of an iron based powder,graphite and a solid particular lubricant having a vaporisingtemperature less the sintering temperature, preferably less than about800° C., and a maximum particle size less than about 0.3 of the maximumsize of the iron based powder as measured by laser diffractionmeasurement, compacting the mixture and sintering the compacted body. 3.A composition comprising an iron based powder, graphite and a solidparticular lubricant having a vaporising temperature less the sinteringtemperature, preferably less than about 800° C., the particle size ofthe lubricant being selected so that the largest pores of a compactedand sintered product prepared from the composition are equal to or lessthan the largest pores obtained in a compacted and sintered productprepared from the composition without lubricant.
 4. A composition of aniron based powder, graphite and a solid particular lubricant having avaporising temperature less the sintering temperature, preferably lessthan about 800° C. and a maximum particle size less than about 0.3 ofthe maximum size of the iron based powder as measured by laserdiffraction measurement.
 5. The composition according to claim 4 whereinthe lubricant has a maximum particle size of at most 60 μm.
 6. Thecomposition according to claim 4 or 5, wherein the maximum particle sizeof the graphite particles is equal to or less than the maximum size ofthe lubricant particles.
 7. The composition according to claim 5 whereinthe iron based powder has a maximum particle size less than 220 μmmeasured by laser diffraction measurement.